SDL screws with the currrent working directory - unless in your code you've changed the CWD to be "x.app/Content/Resources" prior to trying to open the .nib, the CWD will be the directy which contains "x.app" - try moving the .nib there (just a suggestion, as I've never tried using .nibs with SDL.)

N.b. anything in the resources tab in Xcode is copied into "x.app/Content/Resources"